Venous valves are self-closing, one-way valves found within native veins and are used to assist in returning blood back to the heart in an antegrade blood flow direction from all parts of the body. The venous system of the leg for example includes the deep venous system and the superficial venous system, both of which are provided with venous valves that are intended to prevent retrograde flow, which can lead to blood pooling or stasis in the leg. Incompetent valves can also lead to reflux of blood from the deep venous system to the superficial venous system and the formation of varicose veins. Superficial veins which include the greater and lesser saphenous veins have perforating branches in the femoral and popliteal regions of the leg that direct blood flow toward the deep venous system and generally have a venous valve located near the junction with the deep venous system. Deep veins of the leg include the anterior and posterior tibial veins, popliteal veins, and femoral veins. Deep veins are surrounded in part by muscular tissues that assist in generating flow by muscle contraction during normal walking or exercising.
Blood pressure in the veins of the lower leg of a healthy person may range from 0 mm Hg to over 200 mm Hg, depending on factors such as the activity of the body (i.e., stationary or exercising), the position of the body (i.e., supine or standing), and the location of the vein (i.e., ankle or thigh). For example, venous pressure may be approximately 80-90 mm Hg while standing and may be reduced to 60-70 mm Hg during exercise. Despite exposure to such pressures, the valves of the leg are very flexible and can close with a pressure differential of less than one mm Hg.
Veins typically in the leg can become distended from prolonged exposure to excessive blood pressure and due to weaknesses found in the vessel wall. Distension of veins can cause the natural valves therein to become incompetent leading to retrograde blood flow in the veins. Such veins no longer function to help pump or direct the blood back to the heart during normal walking or use of the leg muscles. As a result, blood tends to pool in the lower leg and can lead to leg swelling and the formation of deep venous thrombosis and phlebitis. The formation of thrombus in the veins can further impair venous valvular function by causing valvular adherence to the venous wall with possible irreversible loss of venous function. Continued exposure of the venous system to blood pooling and swelling of the surrounding tissue can lead to post phlebitic syndrome with a propensity for open sores, infection, and may lead to limb amputation.
Chronic venous insufficiency (CVI) occurs in patients that have deep and superficial venous valves of their lower extremities (distal to their pelvis) that have failed or become incompetent due to the aforementioned vessel weakness as well as, for e.g., valve prolapse, congenital valvular abnormalities, such as missing valves, and/or vascular disease that results in valve damage. As a result, such patients may suffer from varicose veins, swelling and pain of the lower extremities, edema, hyper pigmentation, lipodermatosclerosis, and/or deep vein thrombosis (DVT). Such patients are at increased risk for development of soft tissue necrosis, ulcerations, pulmonary embolism, stroke, heart attack, and amputations.
Repair and replacement of venous valves presents a formidable challenge due to the low blood flow rate found in native veins, the very thin and distensible wall structure of the venous wall and the venous valve, and the ease and frequency with which venous blood flow can be impeded or totally blocked for a period of time. Surgical reconstruction techniques used to address venous valve incompetence include venous valve bypass using a segment of vein with a competent valve, venous transposition to bypass venous blood flow through a neighboring competent valve, and valvuloplasty to repair the valve cusps. These surgical approaches may involve placement of synthetic, allograft and/or xenograft prostheses inside of or around the vein. However, such prostheses have not been devoid of problems, such as thrombus formation and valve failure due to the implanted prostheses causing non-physiologic flow conditions and/or excessive dilation of the vessels with a subsequent decrease in blood flow rates.
Percutaneous endoluminal methods for treatment of venous insufficiency are being studied, some of which include placement of synthetic, allograft and/or xenograft valve prosthesis that suffer from similar problems as the surgically implanted ones discussed above. In light thereof, there is still a need in the art for an improved device that may be percutaneously placed within a vein having an existing insufficient, malfunctioning venous valve to re-establish apposition between the valve leaflets to thereby restore proper flow through the vein segment.